The present invention relates to lubricating oil compositions for manual transmissions, particularly to manual transmission lubricating oil compositions with excellent fuel saving properties.
In recent years, energy savings of automobiles, constructions machines, and agricultural machines have been urgently sought as one of the measures for reducing carbon dioxide gas emission from the view point of dealing with environmental issues. Accordingly, lubricating oils used in these machines are required to be more less in stirring resistance and friction resistance than eve before.
Automobile manual transmissions have gear bearing systems. Therefore, the energy saving, i.e., fuel saving of automobiles equipped with such transmissions can be achieved by lowering the viscosity of a transmission lubricating oil because the stirring resistance and friction resistance (friction loss) thereof can be decreased, resulting in an improvement in the power transmission efficiency of the transmissions.
Known lubricating oils for manual transmissions, which have excellent lubricity and low-temperature fluidity, include those with a kinematic viscosity at 40° C. of around 150 mm2/s (see, for example, Patent Document 1 below). However, such known lubricating oils are too high in stirring resistance and friction resistance to contribute to the improvement of fuel efficiency.
On the other hand, when a lubricating oil for manual transmissions is lowered in viscosity, it is drastically deteriorated in anti-seizure properties. The lubricating oil thus causes seizure of not only the gears and bearings of the transmissions but also of the shift forks which determine the gear ratio upon shifting, possibly resulting in vibrations and defects in, for example, the shifting characteristics. It is considered that anti-seizure properties can be improved with the use of a high-viscosity lubricating oil as disclosed in Patent Document 1 below or addition of an extreme pressure additive. It is known that a polysulfide compound is effective in improving the friction properties between synchronizer rings and gear cones (see, for example, Patent Document 2). However, it is found that a low viscosity lubricating oil raises other problems such as deterioration of its anti-wear properties, in particular for preventing shift forks and synchronizer rings determining the shifting characteristics from wearing and also deterioration of shifting characteristics and synchronizing characteristics, depending on the selection of additives or mutual interaction thereof. It is, therefore, necessary to optimize the selection of base oils, extreme pressure additives, and other additives to be blended.
(1) Patent Document 1: Japanese Patent Laid-Open
Publication No. 9-208976
(2) Patent Document 2: Japanese Patent Laid-Open
Publication No. 2001-311090